Ma C M, Nahum A E
Joint Department of Physics, Royal Marsden Hospital and Institute of Cancer Research, Sutton, Surrey, UK.
Phys Med Biol. 1991 Apr;36(4):413-28. doi: 10.1088/0031-9155/36/4/001.
The validity of the Bragg-Gray cavity theory in photon radiation dosimetry for photon energies from 10 keV to 10 MeV has been investigated quantitatively in this paper. A crucial assumption for Bragg-Gray cavity theory to be valid is that the dose from photon interactions in the detector material is negligible. The ratio, Fair, of the absorbed dose resulting from photon interactions in an air cavity, DPA(air), to that in air under the condition of charged-particle equilibrium, DCPE(air), has been used as a parameter to determine if the air cavity can be classified as a Bragg-Gray cavity. Monte Carlo calculated results show that, for monoenergetic photon beams of energies above about 220 keV, the dose ratio, Fair, is smaller than 0.05 for an air cavity of 6 mm thickness and 6 mm diameter in vacuum. Furthermore, it is shown that the Burlin general cavity theory seriously overestimates the departure from Bragg-Gray behaviour. For clinical photon beams the dose ratio, Fair, is 0.29 for a 150 kVp beam and 0.27 for a 240 kVp beam compared to 0.006 for a 60 Co beam if the cavity is placed at a depth of 5 cm in water. This study confirms that typical air-filled ionization chambers cannot be considered to be Bragg-Gray cavities for low- and medium-energy photon radiation.
本文对布拉格 - 格雷空腔理论在10 keV至10 MeV光子能量的光子辐射剂量学中的有效性进行了定量研究。布拉格 - 格雷空腔理论有效的一个关键假设是探测器材料中光子相互作用产生的剂量可以忽略不计。空气腔中光子相互作用产生的吸收剂量DPA(air)与带电粒子平衡条件下空气中的吸收剂量DCPE(air)的比值Fair，已被用作确定空气腔是否可归类为布拉格 - 格雷空腔的参数。蒙特卡罗计算结果表明，对于能量高于约220 keV的单能光子束，在真空中厚度为6 mm、直径为6 mm的空气腔中，剂量比Fair小于0.05。此外，研究表明伯林通用空腔理论严重高估了与布拉格 - 格雷行为的偏差。对于临床光子束，如果空腔置于水中5 cm深处，150 kVp束的剂量比Fair为0.29，240 kVp束的剂量比Fair为0.27，而60Co束的剂量比为0.006。本研究证实，对于低能和中能光子辐射，典型的充气电离室不能被视为布拉格 - 格雷空腔。
